Fix run_as_background_process not be awaited properly causing LoggingContext problems

[MadLittleMods]

Fix run_as_background_process not be awaited properly causing LoggingContext problems

Basically, searching for any instance of run_as_background_process(...) and making sure we wrap the deferred in make_deferred_yieldable(...) if we try to await the result to make it follow the Synapse logcontext rules.

Part of #18905

Dev notes

The problem code was introduced in #17326

Pull Request Checklist

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[MadLittleMods]

As described in the docstring for why we need to do this:

synapse/synapse/metrics/background_process_metrics.py

Lines 231 to 234 in 4367fb2

	Because the returned Deferred does not follow the synapse logcontext rules, awaiting
	the result of this function will result in the log context being cleared (bad). In
	order to properly await the result of this function and maintain the current log
	context, use `make_deferred_yieldable`.

(docstring updated in #18900)

If you're wondering what the problem looks like in practice, see 1668023

[MadLittleMods]

Thanks for the review @reivilibre 🦊